Heavy metal removal using nanoscale zero-valent iron (nZVI): Theory and application

Highlights: • nZVI is able to perform fast and simultaneous removal of different heavy metal ions. • Fast separation and seeding effect of nZVI facilities its application in wastewater. • A novel process of E_h-controlled reactor, nZVI separator and reuse is proposed. • E_h-controlled system and nZVI recirculation increase material efficiency of nZVI. • The process produces stable effluent and is effective in wastewater treatment. - Abstract: Treatment of wastewater containing heavy metals requires considerations on simultaneous removal of different ions, system reliability and quick separation of reaction products. In this work, we demonstrate that nanoscale zero-valent iron (nZVI) is an ideal reagent for removing heavy metals from wastewater. Batch experiments show that nZVI is able to perform simultaneous removal of different heavy metals and arsenic; reactive nZVI in uniform dispersion brings rapid changes in solution E_h, enabling a facile way for reaction regulation. Microscope characterizations and settling experiments suggest that nZVI serves as solid seeds that facilitate products separation. A treatment process consisting of E_h-controlled nZVI reaction, gravitational separation and nZVI recirculation is then demonstrated. Long-term (>12 months) operation shows that the process achieves >99.5% removal of As, Cu and a number of other toxic elements. The E_h-controlled reaction system sustains a highly-reducing condition in reactor and reduces nZVI dosage. The process produces effluent of stable quality that meets local discharge guidelines. The gravitational separator shows high efficacy of nZVI recovery and the recirculation improves nZVI material efficiency, resulting in extraordinarily high removal capacities ((245 mg As + 226 mg-Cu)/g-nZVI). The work provides proof that nanomaterials can offer truly green and cost-effective solutions for wastewater treatment.